drm/amd/powerplay: properly set the dpm_enabled state

On the ASIC powered down(in baco or system suspend),
the dpm_enabled will be set as false. Then all access
(e.g. df state setting issued on RAS error event) to
SMU will be blocked.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>